The present invention relates to a method of producing a nanotube layer on a substrate.
Carbon nanotubes are honeycomb-structured cylindrical tubules of carbon having sp2-structure. Depending on the manufacturing process, distinctions are made between SWNT (Single Wall Nanotubes) or MWNT (Multiwalled Nanotubes). The free-standing ends of the nanotubules are closed under the common synthesis conditions by hemispheres which are realized through inclusion of precisely six pentagonal units in the hexagonal graphite structure. The scientific and, as a result, industrial interest in such nanostructures is based on their superior mechanical and electronic properties. Such structures have a diameter in the nm range and a length up to several μm and find technological application in particular in the field of microelectronics. Currently, great efforts are undertaken to use carbon nanotubes also in the FED-technology.
Although the production of carbon nanotubes has been widely reported, an industrial use of nanotube layers in the emission technology has still not been realized despite many efforts. Various production processes have been proposed including arc discharge at temperatures of about 3000° C., decomposition reactions of SiC wafers at temperatures of 1200° C. under high vacuum, or CVD (chemical vapor deposition) processes, which allow synthesis temperatures for nanotubes even lower than 650° C. Common to all production methods is the term of catalyzed σ-bond metathesis which simply means that the formation mechanism of carbon nanotubes is still speculative. Many experiments confirm catalytic activity in the fullerene/nanotube synthesis of transition metals but also of some elements in the lanthanide group, however, the ideal catalyst or the ideal catalyst composition has not yet been found.
An important issue for technical volume applications of a material is the provision of a reasonable automated process which, especially in the field of microelectronics, should satisfy high clean room conditions.
Nanotube layers are usually produced by separation of carbon atoms from a carbon-containing gas and their deposition on the surface of a substrate. To trigger this decomposition temperature, reaction temperatures in the range between 400 to 2000° C. are needed. However, the actually chosen temperature depends on the substrate material as well as on the used carbon source.
To date, the entire interior of the reaction chamber has to be heated just like a baking oven in order to heat the substrate. This procedure suffers, however, form shortcomings because the heating process does not only affect the substrate but subjects also all other objects placed inside the reaction chamber to heat exposure, in particular the walls of the reaction chamber, so that carbon will deposit thereon as well. As a consequence, undesired carbon deposits contaminate the reaction chamber. Moreover, any energy used for heating objects other than the substrate is considered as loss energy, so that the efficiency of the overall process is adversely affected.
It would therefore be desirable and advantageous to provide an improved method of producing a nanotube layer on a substrate to obviate prior art shortcomings and to substantially reduce formation of undesired carbon layers on objects other than the substrate while significantly reducing heat losses.